Jekyll and Hyde metal's crumbling trick caught on film

Researchers have captured the first video footage of "tin pest" - the transformation of tin from a ductile metal to a flaky, brittle semiconductor (see image, right).

This Jekyll and Hyde transformation between two natural forms, or allotropes, is dependent on temperature. When kept below around 13°C for a period of time, metallic beta-tin morphs into powdery alpha-tin.

Tin pest is of relevance today because the lead-free solder used in electronic connections is almost pure tin. Even a small patch of tin pest could change the ability of solder to conduct electricity.

Pest control

But it is difficult to observe the reaction, which can take several months or even years. Davide Di Maio and Chris Hunt at the National Physics Laboratory in Teddington, UK, speeded up the process by seeding the tin with a cadmium-telluride powder.

The powder has a similar crystal structure to alpha-tin, which encourages the metallic beta-tin to become its fragile alter ego by shoving the atoms into a new arrangement.

When the researchers cooled the seeded metallic tin to -35 °C it flipped into alpha-tin in about 30 hours.

A digital camera attached to a microscope recorded a time-lapse video sequence of the transformation. It clearly shows the process starting from one point in the piece of metal, before spreading outwards as the layers of beta-tin slowly peel back and crumble into alpha-tin.

'Not a sprint'

"We have seen how the transformation progresses - that was the main observation," says Di Maio. He thinks that the same technique could be used to assess the risk of tin pest emerging in any tin alloy, and help find appropriate alloy for particular conditions.

Katsuaki Suganuma at Osaka University, Japan, thinks the technique could help determine which conditions contribute to the transformation. "We need to know what kinds of environments, conditions and alloys affect tin pest in the market," he says.

But Bill Plumbridge at the The Open University points out that speeding up the reaction may not accurately reflect the risks of tin suddenly changing structure in an electronic device.

"The major time component - incubation - is eliminated by the seeding process," he says. "Essentially, it is like grading a marathon on the basis of 400-metre sprinting ability."

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